Semiconductor light emitting device and method

ABSTRACT

A light-emitting device includes: a semiconductor structure formed on one side of a substrate, the semiconductor structure having a plurality of semiconductor layers and an active region within the layers; and first and second conductive electrodes contacting respectively different semiconductor layers of the structure; the substrate comprising a material having a refractive index n&gt;2.0 and light absorption coefficient α, at the emission wavelength of the active region, of α&gt;3 cm −1 . In a preferred embodiment, the substrate material has a refractive index n&gt;2.3, and the light absorption coefficient, α, of the substrate material is α&lt;1 cm −1 .

[0001] This is a continuation-in-part of U.S. patent application Ser.No. 09/469,657 and of U.S. patent application Ser. No. 09/470,540, bothfiled on Dec. 22, 1999, and both assigned to the same assignee as thepresent application.

FIELD OF THE INVENTION

[0002] This invention relates to light emitting semiconductor structuresand methods of making same and, more particularly, to devices andmethods employing group III-V nitride semiconductors and to improvingthe manufacture and the operation thereof.

BACKGROUND OF THE INVENTION

[0003] Light emitting semiconductors which emit in several regions ofthe visible spectrum, for example group III-V semiconductors such asaluminum gallium arsenide and gallium phosphide, have achievedcommercial acceptance for various applications. However, forapplications which require blue or green light, for example green to beused for traffic signal lights or blue for a component of ared-green-blue primary color combination to be used for white lighting,efficient semiconductor light emitters have been sought for shortervisible wavelengths. If such solid state light emitting sources wereavailable at reasonable cost, many lighting applications could benefitfrom the reliability and low energy consumption that characterizesemiconductor operation. Short wavelength devices also hold promise ofproviding increased storage capacity on storage media, due to theability to obtain smaller spot sizes for writing and reading on themedia.

[0004] A type of short wavelength light emitting devices that has directenergy bandgap, and has shown excellent promise, is based on group III-Vnitride semiconductors, which include substances such as GaN, AlN, InN,AlInN, GaInN, AlGaN, AlInGaN, BAlN, BInN, BGaN, and BAlGaInN, amongothers. [These are also sometimes referred to as III-nitridesemiconductors.] An example of a light emitting device of this type isset forth in European Patent Publication EP 0926744, which discloses alight emitting device that has an active region between an n-type layerof III-nitride semiconductor and a p-type layer of III-nitridesemiconductor. An electrical potential applied across the n and p layersof the diode structure causes generation of photons at the active regionby recombination of holes and electrons. The wallplug efficiency of thelight emitting diode (LED) structure is defined by the optical poweremitted by the device per unit of electric power. To maximizeefficiency, both the light generated per watt of drive power and theamount of light exiting from the LED in a useful direction areconsidered.

[0005] As noted in the referenced EP Patent Publication, considerableeffort has been expended in prior art approaches to maximize the lightthat is generated from the active region. The resistance of the p-typeIII-nitride semiconductor layer is much higher than the resistance ofthe n-type III-nitride semiconductor layer. The p-electrode typicallyspans essentially the entire active area to spread current uniformly tothe junction with low electrical resistance. Although this increase insize of the p-electrode may increase the amount of light available fromthe active region, it can decrease the fraction of light that exits thedevice, since much of the light must pass through the p-electrode. Thetransmittance of the p-electrode can be increased by making theelectrode thin or providing it with apertures (see, for examplecopending U.S. patent application Ser. No. 09/151,554, filed Sep. 11,1998), but even these somewhat transmissive electrodes can absorb asignificant amount of light, and their light transmissioncharacteristics tend to trade-off against electrical operatingefficiency, such as by compromising the uniform current density desiredfor the active region.

[0006] As noted in the above-referenced parent applications hereof,because III-nitride substrates are not commercially available, growth ofIII-nitride LEDs is typically implemented on non-lattice matchedsubstrates such as sapphire or silicon carbide.

[0007] In so called “flip-chip” or “epitaxy side-down” configurations,the problem of light transmission through the electrodes is eliminated,as the device is flipped over and mounted with the epitaxy side down sothat the light escapes predominantly through the substrate. The mostcommon substrate is sapphire, but the relatively low refractive index ofsapphire (n˜1.8) limits performance due to index mismatch with theepitaxial region (which has n˜2.4) that results in a “waveguiding”effect on some of the light generated at the active region. As describedfurther hereinbelow, this portion of the light is trapped in theepitaxial region for one or more reflections as a significant fractionof the light energy is lost. Roughening of the sapphire/epilayerinterface can help somewhat, but is of limited effectiveness and canhave other disadvantages. Reference can be made to U.S. Pat. No.6,091,085.

[0008] The above-referenced parent applications hereof describe someexamples of prior art approaches that employ substrates of sapphire orof conductive silicon carbide, and these are summarized briefly asfollows:

[0009] Nakamura et al. U.S. Pat. No. 5,563,422, Inoue et al. EuropeanPatent 091577A1, Kondoh et al. European Patent 0926744A2, and Mensz etal., Electronic Letters 33 (24) pp. 2066-2068, 1997, each discloseIII-nitride LEDs formed on sapphire substrates. Nakamura et al.describes an epitaxy side-up configuration, and the others describeinverted (flip-chip) configurations. All of these approaches tend to beperformance limited by, inter alia, the above-noted effect of a portionof the light being waveguided and trapped because of thesapphire/epitaxy index mismatch.

[0010] The published PCT Application WO96/09653 (Edmond et al.)discloses an embodiment with a vertical injection III-nitride LED on aconducting SiC substrate. A conductive buffer layer is required forOhmic conduction from the III-nitride layers to the SiC substrate. Thegrowth conditions required for a conductive buffer layer limits thegrowth conditions available for subsequent layers and thus restricts thequality of the III-nitride active region layers. Also, the conductivebuffer layer may introduce optical loss mechanisms that limit lightextraction efficiency. Furthermore, the SiC substrate must be doped toprovide high electrical conductivity (p<0.2 Ω-cm) for low seriesresistance. Optical absorption resulting from SiC substrate dopantslimits the light extraction efficiency of the device. These conditionsresult in a trade-off between series resistance and light extractionefficiency and serve to limit the electrical-to-optical power conversionefficiency of the LED.

[0011] Small-area surface-mount LED chips are attractive for manyapplications such as contour strip-lighting and backlighting. For thesedevices, high light-generating capability is not required, but thedevices should be efficient and inexpensive. Therefore, it is desirablefor the devices to be as small as possible, with efficient use of theactive region area. Also, these surface-mount devices should be capableof mounting to a package without the use of wirebonds. ConventionalIII-nitride devices most commonly employ sapphire substrates. Thesesubstrates are relatively inexpensive and suitable for the growth ofgood quality nitride films. Because the substrate is insulating, both pand n contacts are made on the same surface of the LED chip. Thisfacilitates attaching the LED to a submount or package in the flip-chipapproach that was first noted above, such that both p and n bondpads areelectrically interconnected to the external package without wirebonds.However, in addition to the previously mentioned problem of light lossdue to waveguiding, the sapphire substrate is difficult to dice in sizesless than 400×400 um{circumflex over ( )}2, making it difficult tofabricate low-cost surface mount LED chip devices.Scribing-and-breaking, or sawing, the sapphire at these dimensionsresults in undesirably low yields and is unfavorable for high-volumemanufacturing.

[0012] An alternative approach to fabricating III-nitride LEDs employs aconductive SiC substrate, as in the above-summarized Edmond et al. PCTpublished Application. The device growth includes a conductive bufferlayer that enables vertical injection operation. The device has theadvantage that it can be diced to very small dimensions (less than300×300 um{circumflex over ( )}2). A disadvantage is that thevertical-injection device is more difficult to package in a surfacemount fashion since p and n contacts are on opposing surfaces, andwirebonds onto the LED chip are required. Furthermore, as previouslynoted, the doping required for SiC to be conducting results in excessivelight absorption within the chip and limits extraction efficiency.Accordingly, conductive silicon carbide substrates do not offer a viableapproach for manufacture of devices such as III-nitride small-areasurface-mount LED chips.

[0013] It is among the objects of the present invention to provideimproved semiconductor device manufacture and operation, particularly inaddressing the types of problems associated with manufacture andoperation of III-nitride small-area surface-mount LED chips.

SUMMARY OF THE INVENTION

[0014] In our above-referenced copending parent applications (Ser. Nos.09/469,657 and 09/470,540), there is disclosed a III-nitridesemiconductor light-emitting device with an increased light generatingcapability. In an embodiment thereof, a large area (greater than 400×400um²) device, formed on a silicon carbide substrate, has an invertedconfiguration; that is, an epitaxy-side down structure with the lightemitted predominately through the substrate. At least one n-electrodeinterposes the p-electrode metallization to provide low seriesresistance. The p-electrode metallization is opaque, highly reflective,and Ohmic, and provides excellent current spreading. Light absorption inthe p-electrode at the peak emission wavelength of the LED active regionis less than 25% per pass. An intermediate material or submount can beused to provide electrical and thermal connection between the LED dieand the package. The submount material may be Si to provide electronicfunctionality such as voltage-compliance limiting operation, protectionfrom electrostatic discharge (ESD), series-string LED arrays, andfeedback-controlled light output. The entire device, including theLED-submount interface, is designed for low thermal resistance to allowfor high current density operation.

[0015] An approach hereof to fabricating efficient, low-cost small-areasurface-mount LED chips employs a substantially transparent,high-refractive index substrate, that can be diced into small chips. Oneembodiment hereof employs a low light absorption SiC substrate as anextraction window rather than as a conductive substrate. In thisembodiment, both p and n contacts are formed on one surface of the LED,facilitating surface-mount attach. Also, the high refractive index ofthe SiC substrate (n˜2.7) eliminates the waveguide problem imposed bythe use of sapphire, resulting in higher extraction efficiency. SiC canbe easily diced into very small chips, so efficient low-cost manufactureof small devices is facilitated by the present invention.

[0016] Preferably, the index of refraction, n, of the substrate is n>2.0and, more preferably, n>2.3. The light absorption of the substrate, atthe relevant wavelength or small range of wavelengths, is defined by thelight absorption coefficient, α, in the equation I=I_(o)e^(−αd), whereI_(o) is the input light intensity, d is the optical pathlength throughthe absorbing medium, and I is the output light intensity. As usedherein, the light absorption coefficient of a substantially transparentsubstrate is preferably α<3 cm⁻¹ and, more preferably, α<1 cm⁻¹.

[0017] In accordance with a form of the invention, there is provided alight-emitting device, comprising: a semiconductor structure formed onone side of a substrate, the semiconductor structure having a pluralityof semiconductor layers and an active region within the layers; andfirst and second conductive electrodes contacting respectively differentsemiconductor layers of the structure; the substrate comprising amaterial having a refractive index n>2.0 and light absorptioncoefficient α, at the emission wavelength of the active region, of α<3cm⁻¹. In a preferred embodiment of this form of the invention, thesubstrate material has a refractive index n>2.3, and the lightabsorption coefficient, α, of the substrate material is α<1 cm⁻¹.

[0018] In accordance with a form of the technique of the invention,there is disclosed a method for making semiconductor light emittingdevice chips, comprising the following steps: providing a substrate ofsubstantially transparent silicon carbide; forming a semiconductorstructure on one side of the substrate, the semiconductor structurehaving a plurality of semiconductor layers and an active region withinthe layers; applying electrodes to semiconductor layers of the structureto form semiconductor light emitting devices; and dicing the substrateand devices into a plurality of chips. In an embodiment of this form ofthe invention, the step of forming a semiconductor structure having aplurality of layers comprises providing a plurality of layers includingan n-type layer of a III-nitride semiconductor and a p-type layer of aIII-nitride semiconductor on opposing sides of the active region. Inthis embodiment the substrate and devices are sawed into dice ofdimensions less than 400×400 um (and/or area less than 0.16 mm²). In afurther embodiment, the substrate and devices are sawed into dice ofdimensions less than 300×300 um (and/or an area less than 0.09 mm²).Alternatively, in these embodiments, at least one dimension is less than400 um or 300 um, as the case may be. The silicon carbide substrates areparticularly advantageous for this purpose, as compared, for example, tothe more common sapphire substrates which cannot be easily diced intosuch small pieces using presently available techniques withoutunacceptable breakage that substantially lowers yields.

[0019] Further features and advantages of the invention will become morereadily apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTON OF THE DRAWINGS

[0020]FIG. 1 is a diagram that illustrates a prior art III-nitride lightemitting device with a sapphire substrate, which demonstrates thedeleterious waveguiding effect that reduces useful light output.

[0021]FIG. 2 is a diagram of a III-nitride light emitting device inaccordance with an embodiment of the invention, which employs atransparent silicon carbide substrate, and demonstrates the improvedlight extraction that results from the absence of the waveguidingeffect.

[0022]FIG. 3 is a graph that illustrates the advantage of hightransparency silicon carbide as compared to sapphire in producing a highefficiency III-nitride light emitting device.

[0023]FIG. 4 is a flow diagram illustrating steps of an LED chipfabrication technique in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[0024]FIG. 1 illustrates an example of a prior art type of III-nitridesemiconductor device 110 in a so-called flip-chip or inverted orepitaxy-down configuration. In this example, the substrate 115, on whichthe device was originally formed, is sapphire, which has an index ofrefraction n˜1.8. [The substrate 115, after inversion, is sometimesreferred to as a superstrate.] The epitaxy region 120 of III-nitridesemiconductor layers includes n-type layer 122 and p-type layer 128,with an active region p-n junction between the n-type and p-type layers.Typically, an active region layer 125 of III-nitride semiconductor isdisposed at the p-n junction. A conductive p-electrode 138 is depositedon the p-type layer 128 and a conductive n-electrode 132 is deposited onthe n-type layer 122. In this prior art configuration, the electrodesare typically opaque and reflective, and the objective is to emit asmuch of the light as possible through the top and sides of the sapphiresuperstrate 115. The sapphire is relatively transmissive of the opticalradiation. However, sapphire has an index of refraction, n˜1.8, which issubstantially lower than the index of refraction of the epilayer region(n˜2.4). This results in a critical angle of about sin θ_(c)=1.8/2.4;that is, θ_(c)=48.6 degrees. Accordingly, any portion of the light thatis incident on the sapphire/epilayers boundary at an angle which isgreater than the critical angle (with respect to the normal to theboundary), will experience total internal reflection, and thiscorresponds to roughly 70% of the total generated light from the activeregion. This results in the “waveguiding” type of effect illustrated forthe ray 141 in FIG. 1, and substantial loss of output light, since eachreflection involves some loss of light energy, particularly reflectionsfrom lossy media such as the metal electrodes and lossy transmissionthrough the epilayers. Accordingly, it is seen that lightextraction-from conventional flip-chip III-nitride LEDs, which employsapphire superstrates, is problematic due, at least in part, towaveguiding in the III-nitride epilayers that results in losses ofprecious light energy.

[0025] The improvement in light extraction resulting from an embodimentof the invention is illustrated in conjunction with FIG. 2, which againshows a flip-chip III-nitride semiconductor LED, designated 210, but inthis case with a substantially transparent high index substrate 215. Ina preferred embodiment hereof, the substrate material comprisessubstantially transparent silicon carbide. The epitaxy region 220 ofIII-nitride semiconductor layers includes n-type layer 222 and p-typelayer 228, with a p-n junction between the n-type and p-type layers. Anactive region 225 of III-nitride semiconductor is disposed at the p-njunction. In one example, the p- and n-type layers include p-type GaN(such as GaN:Mg) and n-type GaN (such as GaN:Si), while the activeregion is comprised of InGaN, and may include one or more InGaN quantumwell layers. However, any suitable material system may be used. Aconductive p-electrode 238 is deposited on the p-type layer 228 and aconductive n-electrode 232 is deposited on the n-type layer 222. Again,the electrodes are opaque and reflective. In this case, the transparentSiC has an index of refraction n˜2.7, which is greater than the epilayerindex (n˜2.4). This removes the critical angle consideration, andeliminates the type of waveguiding within the epilayer region that wasseen in the FIG. 1 example. The low light absorption of the superstrateat the emission wavelength also contributes substantially to the outputefficiency. The light absorption coefficient, α, at the emission peakwavelength, is preferably α<3 and, more preferably, α<1. In thisinstance, the majority of the light incident on the interface betweenthe epilayer region and the superstrate (generally either emanatingdirectly from the active region or after a single reflection from areflective electrode) will pass into the superstrate 215, with the highprobability to become useful output light. As seen in FIG. 2, it can beadvantageous in this embodiment to provide rough surfaces (such as sawnedges or ground top surfaces) on the superstrate to help light escapefrom the superstrate and into the lower index environment outside thesuperstrate; that is, either the air or an encapsulant having anintermediate index of refraction.

[0026]FIG. 3 is a graph that illustrates the advantage of hightransparency silicon carbide as compared to sapphire in producing a highefficiency III-nitride light emitting device. Extraction efficiency,normalized to the case of GaN/sapphire (FIG. 1), is plotted as afunction of SiC absorption coefficient, for different thicknesses ofSiC. These calculations were made for the example of anepoxy-encapsulated chip. The dependence of extraction efficiency onabsorption coefficient will increase in the case of a non-encapsulatedchip.

[0027]FIG. 4 is a flow diagram which describes a technique forpracticing an embodiment of the invention for fabricating small-area LEDchips. The block 410 represents the providing of a suitablesubstantially transparent high refractive index substrate. As aboveindicated, the substrate is selected to have a light absorptioncoefficient, α, at the emission peak wavelength, preferably α<3 cm⁻¹,and, more preferably α<1 cm⁻¹, and to have an index of refraction, n,preferably n>2.0 and, more preferably n>2.3. In one preferred embodimenthereof, the substrate is transparent silicon carbide having an index ofrefraction n˜2.7, and an absorption coefficient, α, less than about 1cm⁻¹ in the ˜470 nm wavelength regime. In an example hereof, the SiCsubstrate has a thickness in the range 50 to 500 um. The block 420represents the deposition of the III-nitride epilayers, in knownfashion, on the substrate. Next, as represented by the block 430, themetalizations, including the electrodes and bondpads, are deposited. Thesubstrate wafer can then be diced into chips, either byscribing-and-breaking or using a sawing technique (block 440). In oneembodiment hereof, the chips are sawed into dice of dimensions less than400×400 um (and/or area less than 0.16 mm²). In a further embodimenthereof, the chips are sawed into dice of dimensions less than 300×300 um(and/or an area less than 0.09 mm²). Alternatively, in theseembodiments, at least one dimension is less than 400 um or 300 um, asthe case may be. The silicon carbide substrates are particularlyadvantageous for this purpose, as compared, for example, to sapphiresubstrates which cannot be easily diced into such small pieces usingpresently available techniques without unacceptable breakage thatsubstantially lowers yields. The preferred surface roughening on theexposed surfaces of the substrate, not already suitably roughened bysawing, can be achieved, for example, by grinding or etching. Next, asrepresented by the block 450, the chips are attached to submounts (thisstep being optional), and final packaging can then be implemented (block460).

[0028] In accordance with a further embodiment of the invention, thesubstrate can comprise transparent polycrystalline silicon carbide,which can be formed by chemical vapor deposition (CVD) and machined intowafers of large sizes and arbitrary shapes. Because the polycrystallinegrains are oriented, these substrates could be suitable for MOCVD growthusing, for example, the III-nitride material system. Furthermore, alow-impurity version of such a substrate could provide the requiredattributes (n>2.0, α<3 cm⁻¹) for a high-efficiency LED as describedherein. In another form of this embodiment, the polycrystalline SiCwafers can be laminated with a single crystal 3C SiC film via asilicon-on-insulator (SOI) wafer-bonding and carbonization process.[Reference can be made to K. D. Hobart et al., J. Electrochem. Soc.146,3833-3836, 1999.] The laminated substrates can be used to grow theIII-nitride epitaxial layers, as in the previously described embodiment,and devices can be formed thereon before dicing into small chips andbonding to submounts before final packaging to form the small areasurface-mount LED chips. These devices have the previously enumeratedadvantages, including the ability during manufacture to to be sawn intosmall chips, and the operational advantage of substantially eliminatingthe trapped light losses that resulted in the prior from the refractiveindex mismatch between the III-nitride epilayers and a sapphiresuperstrate. Furthermore, the relatively low cost large-areapoly-SiC/SiC substrates can provide high volume production advantages.Low absorption GaN, AlN, or other III-nitride substrates can also beutilized.

1. A light-emitting device, comprising: a semiconductor structure formedon one side of a substrate, said semiconductor structure having aplurality of semiconductor layers and an active region within saidlayers; and first and second conductive electrodes contactingrespectively different semiconductor layers of said structure on saidone side of said substrate; said substrate comprising a material havinga refractive index n>2.0 and light absorption coefficient α, at theemission wavelength of said active region, of α<3 cm⁻¹.
 2. The device asdefined by claim 1, wherein said substrate material has a refractiveindex n>2.3.
 3. The device as defined by claim 1, wherein said lightabsorption coefficient α, of said substrate material is α<1 cm⁻¹.
 4. Thedevice as defined by claim 2, wherein said light absorption coefficientα, of said substrate material is α<1 cm⁻¹.
 5. The device as defined byclaim 1, wherein said substrate material comprises silicon carbide. 6.The device as defined by claim 4, wherein said substrate materialcomprises silicon carbide.
 7. The device as described by claim 1,wherein said substrate material comprises polycrystalline siliconcarbide.
 8. The device as described by claim 4, wherein said substratematerial comprises polycrystalline silicon carbide.
 9. The device asdefined by claim 1, wherein said substrate material comprisespolycrystalline silicon carbide having single crystal silicon carbidethereon.
 10. The device as defined by claim 4, wherein said substratecomprises polycrystalline silicon carbide having single crystal siliconcarbide thereon.
 11. The device as defined by claim 5, wherein saiddevice has at least one dimension perpendicular to its thickness that isless than 400 um.
 12. The device as defined by claim 5, wherein saiddevice has dimensions perpendicular to its thickness that are less than400×400 um.
 13. A light emitting device, comprising: a semiconductorstructure formed on one side of a substrate, said structure having aplurality of semiconductor layers and an active region within saidlayers, said plurality of layers including an n-type layer of aIII-nitride semiconductor and a p-type layer of III-nitridesemiconductor on opposing sides of said active region; first and secondconductive electrodes respectively contacting said n-type layer and saidp-type layer; and means for applying electric signals across saidelectrodes to produce light at said active region, the majority of saidlight being emitted from said device via said substrate; said substratecomprising a material having a refractive index n>2.0 and lightabsorption coefficient α, at the emission wavelength of said activeregion, of α<3 cm⁻¹.
 14. The device as defined by claim 13, wherein saidsubstrate material has a refractive index n>2.3.
 15. The device asdefined by claim 13, wherein said light absorption coefficient α, ofsaid substrate material is α<1 cm⁻¹.
 16. The device as defined by claim14, wherein said light absorption coefficient α, of said substratematerial is α<1 cm⁻¹.
 17. The device as defined by claim 14, whereinsaid substrate material comprises silicon carbide.
 18. The device asdefined by claim 15, wherein said substrate material comprises siliconcarbide.
 19. The device as defined by claim 16, wherein said substratematerial comprises silicon carbide.
 20. The device as described by claim14, wherein said substrate material comprises polycrystalline siliconcarbide.
 21. The device as defined by claim 14, wherein said substratematerial comprises polycrystalline silicon carbide having single crystalsilicon carbide thereon.
 22. The device as defined by claim 13, whereinsaid substrate material comprises GaN.
 23. The device as defined byclaim 13, wherein said substrate material comprises AlN.
 24. The deviceas defined by claim 13, wherein said substrate material comprisesIII-nitride material.
 25. The device as defined by claim 13, whereinsaid device has at least one dimension perpendicular to its thicknessthat is less than 400 um.
 26. The device as defined by claim 16, whereinsaid device has at least one dimension perpendicular to its thicknessthat is less than 400 um.
 27. The device as defined by claim 14, whereinsaid device has dimensions perpendicular to its thickness that are lessthan 400×400 um.
 28. The device as defined by claim 14, wherein saiddevice has an area less than 0.16 mm².
 29. A method for makingsemiconductor light emitting device chips, comprising the steps of:providing a substrate of substantially transparent silicon carbide;forming a semiconductor structure on one side of said substrate, saidsemiconductor structure having a plurality of semiconductor layers andan active region within said layers; applying electrodes tosemiconductor layers of said structure on said one side of saidsubstrate to form semiconductor light emitting devices; and dicing saidsubstrate and devices into a plurality of chips.
 30. The method asdefined by claim 29, further comprising bonding the electrodes of eachdevice chip to a submount to obtain inverted semiconductor lightemitting devices.
 31. The method as defined by claim 29, wherein saidstep of dicing includes sawing of said substrate and devices into aplurality of chips.
 32. The method as defined by claim 29, wherein saidstep of providing a substrate of transparent silicon carbide comprisesproviding a silicon carbide substrate having a refractive index n>2.3and a light absorption coefficient α, at the emission wavelength of saidactive region, of α<3 cm⁻¹.
 33. The method as defined by claim 29,wherein said step of providing a substrate comprises providing asubstrate of polycrystalline silicon carbide.
 34. The method as definedby claim 29, wherein said step of providing a substrate comprisesproviding a substrate of polycrystalline silicon carbide having singlecrystal silicon carbide thereon.
 35. The method as defined by claim 29,wherein said step of dicing into chips comprises dicing into chipshaving at least one dimension perpendicular to thickness that is lessthan 400 um.
 36. The method as defined by claim 32, wherein said step ofdicing into chips comprises dicing into chips having at least onedimension perpendicular to thickness that is less than 400 um.
 37. Themethod as defined by claim 29, wherein said step of dicing into chipscomprises dicing into chips having dimensions perpendicular to thicknessthat are less than 400×400 um.
 38. The method as defined by claim 29,wherein step of dicing into chips comprises dicing into chips having anarea less than 0.16 mm².
 39. The method as defined by claim 29, whereinsaid step of dicing into chips comprises dicing into chips having atleast one dimension perpendicular to thickness that is less than 300 um.40. The method as defined by claim 29, wherein said step of dicing intochips comprises dicing into chips having dimensions perpendicular tothickness that are less than 300×300 um.
 41. The method as defined byclaim 29, wherein step of dicing into chips comprises dicing into chipshaving an area less than 0.09 mm².
 42. The method as defined by claim29, wherein said step of forming a semiconductor structure having aplurality of layers comprises providing a plurality of layers includingan n-type layer of a III-nitride semiconductor and a p-type layer of a111-nitride semiconductor on opposing sides of said active region. 43.The method as defined by claim 32, wherein said step of forming asemiconductor structure having a plurality of layers comprises providinga plurality of layers including an n-type layer of a III-nitridesemiconductor and a p-type layer of a III-nitride semiconductor onopposing sides of said active region.
 44. The method as defined by claim39, wherein said step of forming a semiconductor structure having aplurality of layers comprises providing a plurality of layers includingan n-type layer of a III-nitride semiconductor and a p-type layer of aIII-nitride semiconductor on opposing sides of said active region.
 45. Alight emitting device, comprising: a semiconductor structure formed onone side of a substrate, said structure having a plurality ofsemiconductor layers and an active region within said layers, saidplurality of layers including an n-type layer of a III-nitridesemiconductor and a p-type layer of III-nitride semiconductor onopposing sides of said active region; first and second conductiveelectrodes respectively contacting said n-type layer and said p-typelayer; and means for applying electric signals across said electrodes toproduce light at said active region, the majority of said light beingemitted from said device via said substrate; said substrate comprisingpolycrystalline silicon carbide.
 46. The device as defined by claim 45,wherein said substrate comprises polycrystalline silicon carbide havingsingle crystal silicon carbide thereon.